Fracture Growth in Layered and Discontinuous MediaFracture Growth in Layered and Discontinuous Media . Norm Warpinski Pinnacle – A Halliburton Service . The statements made during

Fracture Growth in Layered and

Discontinuous Media

Norm Warpinski

© 2011 Halliburton. All Rights Reserved. 2

Fracture growth in complex media

Extensiv

In situ stresses

Material properties

Interfaces

Layering

Fracture toughness

Heterogeneities

e research into affects of important parameters

Core Photo

Projected

Borehole FMS

Image

F11F10

F9

F8F7

F6F5

F4

F3F2

F1

4675

4676

4677

4678

2-1/2 in.

Core dia.

DOE/GRI M-Site core through

Mounds Drill Cuttings

Injection Experiment


Hydraulic Fracture Growth

In Situ Stress

Dominant factor in

controlling hydraulic

fracture growth

Mineback tests showing

fracture termination at high

stress layer

DOE Mineback results showing effects of in situ stress contrasts


Fracture Height Growth

In situ stress

Example equilibrium calculation

Requires:

Stress

Pressure

Fracture toughness

In general, more complex

equations are used

Modulus

Layers

2/sin

2 1122

H

K

H

hP Ic

h HP

1 – stress in reservoir

2 – stress in bounding layers

P – pressure in fracture

H – fracture height

h – reservoir thickness

0

100

200

300

400

500

600

700

0 200 400 600 800 1000

He

igh

t (ft

)

Pressure (psi)

Kic=1500

Kic=5000

Simonson et al., SPE, 1978


Modeling fracture growth

Calibrated

models using

diagnostics

Verify data

and behavior

Example in the

Bossier

sandstone in

East Texas

11700

11800

11900

12000

12100

12200

12300

12400

12500

12600

12700

12800

12900

13000

13100

13200

13300

-80

0

-70

0

-60

0

-50

0

-40

0

-30

0

-20

0

-10

0 0

10

0

20

0

30

0

40

0

50

0

60

0

70

0

80

0

Distance Along Fracture (ft)

MD

(ft

)

9:05-10:08am

10:08-11:05am

11:05-12:13pm

perfs

Late events after

net pressure drop

Minor fracturing in

York; not modeled

Frac model

geometry

11700

11800

11900

12000

12100

12200

12300

12400

12500

12600

12700

12800

12900

13000

13100

13200

13300

-80

0

-70

0

-60

0

-50

0

-40

0

-30

0

-20

0

-10

0 0

10

0

20

0

30

0

40

0

50

0

60

0

70

0

80

0

Distance Along Fracture (ft)

MD

(ft

)

9:05-10:08am

10:08-11:05am

11:05-12:13pm

perfs

9:05-10:08am

10:08-11:05am

11:05-12:13pm

perfs

Late events after

net pressure drop

Minor fracturing in

York; not modeled

Frac model

geometry

12100

12200

12300

12400

12500

12600

12700

0 150Gamma R...

Logs : D-14 Gr ...

Rocktype Stress (... Modulu...0 1Permea...

0 200Compo...

FracproPT Lay er Properties

Sand/Shale

Cotton Va...

Cotton Va...

Cotton Va...

Cotton Va...

Cotton Va...

Shale Lo...

Shale Lo...

Shale Lo...

100 200 300 400 500 600 700

Concentration of Proppant in Fracture (lb/f t²)

0 0.07 0.14 0.21 0.28 0.35 0.42 0.49 0.56 0.63 0.70

Proppant Concentration (lb/ft²)

Griffen et al., SPE 84489



Modulus variations

Limited effect due to

influence on width

Van Eekelen

formulation

Interfacial effect

Fracture toughness

behavior at interfaces

Not observed in field

Interface

Low-modulus

material

High-modulus

material

Propagation

Direction

Fracture

Mineback example showing behavior of

fracture at a material property interface

21

2

1

1

2 134

1log

19

121

2 h

H

G

G

h

H

G

GHL



Fracture toughness

Generally assumed to

have a small effect

Relatively low fracture

toughness for rocks

Potential for scale

effects that might

constrain growth

(Shlyapobersky)

4000

5000

6000

7000

8000

9000

0 500 1000 1500 2000 2500D

epth

(ft)

Fracture Toughness (psi- in)

Sandstones

Non-reservoir lithologies

dyyH

yHyp

HK

H

HI

2/

2/

2/

1 2/

2/

Equation for calculating stress intensity factor, Rice (in Fracture, Liebowitz, 1968)

Data from DOE Multiwell experiment in

Piceance basin, Colorado



Composite layering

Mineback photos

suggest a wide variety

of mechanisms are

interplaying

Fracture diagnostics

have shown the same

behavior

Microseismic

Downhole tiltmeters

~ 2 ft



Observed layering

effects at DOE/GRI

M-Site

Microseismic

and downhole

tiltmeter

measurements of

fracture height

In situ stress

measurements

Treatment

pressure

4200

4300

4400

-500 -250 0 250 500

ALONG FRACTURE LENGTH (ft)

DE

PT

H (

ft) MWX-2

C SAND

FRAC 5C480 bbl

Borate

Gel

Minifrac

3500

4000

4500

0 10 20 30 40TIME (min)

PR

ES

SU

RE

(p

si)

Pressure Well

Above Upper

Bounding Stresses

4100

4300

4500

3000 3500 4000 4500

STRESS (psi)

DE

PT

H (ft

)

C SAND

Small Stress

Contrast

Above


Discontinuities

Fracture growth across

discontinuities in the rock

mass has been extensively

studied

Depends upon

Stress

Material properties

Angle of approach

Models

Mineback

Laborataory

Propagation

Cement

Fracture

Jeffrey & Zhang,

2009, SPE 119351

DOE mineback tests


Discontinuities

Fracture behavior as

influenced by a wide range of

discontinuities have been

observed in minebacks and

other tests

Faults

Cleats

Fracture

Strands

Coal

DOE mineback tests

Bureau of Mines Report 9083, Diamond & Oyler, SPE 22395, Diamond CBM Symposium 11/87, Lambert SPE 15258


Hydraulic Fracture Growth

Summary

Hydraulic fractures influenced by heterogeneities

within the reservoir

Any change in properties/uniformity

In situ stress is the dominant influence

Large stress contrasts contain fractures

Layering and interfaces result in inefficient growth

Models available to simulate/mimic behavior

Fracture Growth in Layered and Discontinuous Media Norm Warpinski

Pinnacle – A Halliburton Service

The statements made during the workshop do not represent the views or opinions of EPA. The claims made by participants have not been verified or endorsed by EPA.

Fracture behavior in the vicinity of layered and discontinuous rock masses has been the subject of numerous papers. The major factors that have been investigated are stress variations, modulus variations, fracture toughness variations, interface properties, high permeability zones, combined layering and interfacial behavior, and fluid pressure gradient changes. Of these, stress changes are clearly the largest influence on fracture growth across layers and stress bias is clearly the largest factor in the development of complexity in discontinuous media. Nevertheless, many of the other factors play a significant role in cases where the stress contrasts are not large and in the general development of complex fractures.

In Situ Stress

The in situ stress contrasts clearly have the most significant effect on fracture height growth. The importance of stress was recognized early on (e.g., Perkins and Kern 1961) and has been extensively studied in modeling (e.g., Simonson et al. 1978, Voegele et al. 1983, Palmer and Luiskutty 1985), mineback tests (Warpinski et al. 1982), and numerous laboratory experiments. Fracture height growth can be easily restricted if the layers above and below have higher stress than the reservoir rock, and this is a common occurrence in sedimentary basins. An equilibrium (static) analysis of the Linear Elastic Fracture Mechanics behavior of a fracture surrounded by rocks with higher stress was first given by Simonson et al. (1978) for a symmetric case (stresses above and below are equal). Given the geometry in Figure 9, an equation can be written as

where P is the net pressure in the fracture, 1 is the stress

in the pay zone, 2 is the stress in the bounding layers, h is the thickness of the pay zone, H is the total fracture height, and KIc is the fracture toughness of the bounding layers. In this equation, the first term on the right is due to the stress contrasts, while the second term is due to fracture toughness. For standard laboratory values of fracture toughness, the term on the left is generally small (unless the fracture is very small) and the height of the fracture is mostly dependent on the stress contrasts. In general, this equation is

2/

sin2 1

122H

K

H

hP Ic

Figure 9. Geometry for stress effects.

conservative since there are other dynamic factors that affect the amount of height growth that will occur. Similar equations can be developed for non-symmetric stress contrasts, but more complete dynamic analyses are usually performed in fracture models.

Layer Material Property Differences

While Simonson et al. (1978) show that a material property interface in an ideal situation could blunt fracture growth, years of fracturing experience (Nolte and Smith 1979), fracture diagnostic monitoring (Warpinski et al. 1998, Wright et al. 1999), mineback testing (Warpinski et al. 1982), and other research (Smith et al. 1982; Teufel and Clark 1984; Palmer and Sparks 1990) have shown that this is not the case. Figure 10 shows an example of a dyed water fracture that has propagated through an interface from a low modulus material into a high modulus material (Warpinski et al. 1982). A more complete discussion of the role of the interface has been given by Cleary (1978), where the complexities of the interface, the micromechanics of the fracturing process, the potential for blunting and twisting (no longer only mode I fracture

growth), and various other factors make the problem difficult to analyze with standard analysis tools. What is clear from these studies is that crossing interfaces requires additional energy and can hinder vertical growth. Modulus contrasts clearly have an effect on the width of the fracture and can be expected to enhance or restrict fluid flow appropriately. Cleary (1980) provided a time-constant analysis of the effect of modulus, while Van Eekelen (1980) developed a relationship based on relative height changes in the layers, given by

21

2

1

1

2 134

1log

19

121

2

h

H

G

G

h

H

G

GHL .

As discussed by Van Eekelen (1980) and Smith et al. (2001), these effects are generally small and cannot be expected to provide significant containment of fractures. Gu and Siebrits (2008) also show that low modulus layers surrounding a higher modulus pay zone can be restrictive due to a lowered stress intensity factor, but this also depends on the relative fracture toughness of the different materials.

Figure 10. Mineback photo of fracture propagating across interface.

Fracture Toughness

Fracture toughness can have a very significant impact on fracture growth, and a large value of KIc can either induce a high pressure, restrict the height, or both. For a homogeneous formation, the stress intensity factor at the top of the fracture can be computed if the net stress distribution is known by

Laboratory experiments have generally shown that fracture toughness varies over only a limited range (e.g., Hsiao and El Rabaa 1987), which suggests that fracture toughness effects will be rather limited. Figure 11 shows a compendium of fracture toughness measurements made at the DOE MWX experiment that shows the relatively small range for both reservoir and non-reservoir rocks. However, the scale dependence of fracture toughness (or potentially other types of tip effects) is not well understood for large scale fractures, so there may be potential for fracture containment due to this mechanism (Shlyapobersky et al 1998).

Interfaces

It is well known that weak interfaces can blunt fracture growth, and such a mechanism is often cited for the use of KGD (Khristianovich, Geertsma and De Klerk) models (Nierode 1985). Examples of blunting have been noted in mineback experiments (Warpinski et al 1982, Warpinski and Teufel 1987, Jeffrey et al. 1992, Zhang et al. 2007) and laboratory experiments (Anderson 1981, Teufel and Clark 1984). While it is generally expected that weak interfaces will be most important at shallow depths where friction due to the overburden stress is a minimum, other factors such as overpressuring or embedded particulates (equivalent to a fault gouge) can clearly minimize frictional effects even at great depths. Weak interfaces have the potential of totally stopping vertical fracture growth, initiating interface fractures, or causing offsets in the fracture. In addition to restricted growth effects, weak interfaces above and below the reservoir can decouple the fracture walls (Barree and Winterfeld 1998, Gu et al. 2008), resulting in poor coupling of the fracture pressure in the reservoir to the fracture outside of the weak

dyyH

yHyp

HK

H

HI

2/

2/

2/

1 2/

2/ ,

where p(y) is the net stress distribution vertically. If the stress intensity factor exceeds the fracture toughness of the material, the fracture will propagate. Obviously, the situation becomes more complex (and not analytic) for layered materials with different elastic properties, but the equation above gives a rough estimate of the fracture stability.

4000

5000

6000

7000

8000

9000

0 500 1000 1500 2000 2500

Dep

th (f

t)

Fracture Toughness (psi-in)

Sandstones

Non-reservoir lithologies

Figure 11. Fracture toughness data from MWX.

interfaces. This reduced coupling would create narrower fractures in the layers across the interface and much wider fractures within the reservoir rock. Many mechanism, such as those described above and others, can be bundled together to describe fracturing across a succession of interfaces. The possibility that such layered media could contain hydraulic fractures has been derived from fracture diagnostic information (Warpinski et al. 1998, Wright et al. 1999, Griffin et al. 1999). It is easy to conceive of multiple mechanisms serving to blunt, kink, offset, bifurcate, and restrict growth in various layers, much as a composite material hinders fracture growth across it. Various methods are now being used to model such behavior (Wright et al. 1999, Miskimmins and Barree 2003, Weijers et al. 2005). Several of the mechanisms can be seen in Figure 12, which is a mineback photo of a fracture propagating upward across several interfaces. The left-hand side is the unaltered photograph, while the right-hand side has the fracture accentuated with a line drawn over it. There is kinking, offsetting, and bending occurring as the fracture makes its way through the layers. In other cases, additional fractures are initiated or some fractures are terminated.

Figure 13 shows a schematic of several types of behavior that have been observed in minebacks or laboratory tests. The result of these behaviors could be any combination of complexity, restriction, or termination of the fracture as it propagates across the layered medium. Restrictions should be common if kinking or offsets occur, as the width in the

~ 2 ft

Figure 12. Photograph and line drawing of fracture behavior crossing interfaces.

Figure 13. Schematic of types of observed fracture behavior crossing interfaces.

kink or offset will necessarily be less than in the vertical part of the fracture due to both geometric and stress considerations.

(Warpinski et al. 1993, Branagan et al 1996) and mineback tests. They prevent fractures from propagating as a single planar feature and instead force it into multiple, variably connected, intersecting components. This complexity makes it difficult for fractures to grow large distances as planar features.

High permeability interval

High permeability zones can also terminate vertical fracture growth by dehydrating the slurry through high leakoff. Coals are excellent examples of zones where fracture growth might be terminated by this mechanism.

Summary

Hydraulic fracture growth is influenced by a multiplicity of factors that are common in any reservoir. Of most importance is the in situ stress distribution, but interfaces, natural fractures, and other heterogeneities may also significantly affect behavior.

Discontinuities

Any heterogeneities and discontinuities can modify thpropagation behavior of fractures in a rock mass. Figur14 shows an example of a fracture that is crossing unhealed natural fractures (Warpinski et al. 1981), whiis also equivalent to the case of a weak interface with some permeability along the interface. This example shows offsets of the fractures at a location that is very close to the wellbore. Cement was used as the fracturifluid for this test in order to preserve the width of the fracture. Such offsets would clearly restrict fracture growth because of the narrower width of the fracture ithe offset and the possibility of sand bridging. There have been many studies of the factors that influence fracture growth across discontinuities (e.g., Teufel 1979). These studies have demonstrated the effects of stress, angle of approach, and various materiproperties in blunting or offsetting fractures. These tyof offsets are likely responsible for much of the complexity observed in hydraulic fractures in cores

p

e e

ch

ng

n

al es Figure 14. Fracture crossing

discontinuities.

References

Anderson, G.D. 1981. Effects of Friction on Hydraulic Fracture Growth near Unbonded Interfaces in Rocks. SPEJ 21:21-29.

Baree, R.D. and Winterfeld, P.H. 1998. Effects of Shear Planes and Interfacial Slippage on Fracture Growth and Treating Pressures. Paper SPE 48926 presented at the SPE Annual Technical Conference and Exhibition. New Orleans, Louisiana, 27-30 September.

Branagan, P, Peterson, R, Warpinski, N, and Wright, T. 1996. Results of Multi-Site Project Experimentation in the B-Sand Interval: Fracture Diagnostics and Hydraulic Fracture Intersection. Gas Research Institute Report GRI-96/0225, Chicago, Illinois.

Cleary, M.P. 1978. Primary Factors Governing Hydraulic Fractures in Hetrogeneous Stratified Porous Formations. Paper 78-Pet-47 presented at the 1978 ASME ETC Conference. Houston, Texas, Nov 5-9.

Cleary, M.P. 1980. Analysis of Mechanisms and Procedures for Producing Favourable Shapes of Hydraulic Fractures. Paper SPE 9260 presented at the 55th SPE Annual Fall Technical Conference and Exhibition. Dallas, Texas. 21-24 September.

Griffin, L.G., Wright, C.A., Davis, E.J., Weijers, L., and Moschovidis, Z.A. 1999. Tiltmeter Mapping to Monitor Drill Cuttings Disposal. Proceedings of the 37th Annual Rock Mechanics Symposium. Vail, Colorado. 2:1033-1040, 6-9 June.

Gu, H. and Siebrits, E. 2008. Effect of Formation Modulus Contrast on Hydraulic Fracture Height Containment. SPE Production and Operations 23:2-170-176.

Gu, H., Siebrits, E. and Sabourov, A. 2008. Hydraulic-Fracture Modeling with Bedding Plane Interfacial Slip. Paper 2008 presented at the SPE Eastern Regional/AAPG Eastern Section Joint Meeting. Pittsburgh, Pennsylvania. 11-15 October.

Hsiao, C. and El Rabaa, A.W. 1987. Fracture Toughness Testing of Rock Cores. Presented at the 28th U.S. Symposium on Rock Mechanics. 141-148. Tucson, Arizona, 29 June – 1 July.

Jeffrey, R.G., Byrnes, R.P., Lynch, P.A. and Ling, D.J. 1992. An Analysis of Hydraulic Fracture and Mineback Data for a Treatment in the German Creek Coal Seam. Paper SPE 24362 presented at the SPE Rocky Mountain Regional Meeting. Casper, Wyoming, 18-21 May.

Miskimmins, J.L. and Barree, R.D. 2003. Modeling of Hydraulic Fracture Height Containment in Laminated Sand and Shale Sequences. Paper SPE 80935 presented at the SPE Production Operations Symposium, Oklahoma City, Oklahoma, 22-25 March.

Nierode, D.E. 1985. Comparison of Hydraulic Fracture Design Methods to Observed Field Results. JPT 37:1831-1839.

Nolte, K.G. and Smith, M.B. 1981. Interpretation of Fracturing Pressure. JPT 33:1767-1775. Palmer, I.D. and Luiskutty, C.T. 1985. A Model of the Hydraulic Fracturing Process for Elongated

Vertical Fractures and Comparisons of Results with Other Models. Paper SPE 13864 presented at the SPE/DOE Low Permeability Gas Reservoirs Symposium. Denver, Colorado, 19-22 May.

Palmer, I.D. and Sparks, D.P. 1991. Measurement of Induced Fractures by Downhole TV Camera in Black Warrior Basin Coalbeds. JPT 43:270-275; 326-328.

Perkins, T.K. and Kern, L.R. 1961. Widths of Hydraulic Fractures. JPT 13:937-949. Shlyapobersky, J., Issa M.A., Issa, M.A., Islam, M.S., Dudley, J.W., Shulkin, Y. and Chudnovsky, A.

1998. Scale Effects on Fracture Growth Resistance in Poroelastic Media. Paper SPE

48929 presented at the SPE Annual Technical Conference and Exhibition. New Orleans, Louisiana, 27-30 September.

Simonson, E.R., Abou-Sayed, A.S., and Clifton, J.J. 1978. Containment of Massive Hydraulic Fractures. SPEJ 18:27-32.

Smith, M.B., Rosenberg, R.J. and Bowen, J.F. 1982. Fracture Width – Design vs. Measurement. Paper SPE 10965 presented at the SPE Annual Technical Conference and Exhibition. New Orleans, Louisiana, 26-29 September.

Smith, M.B., Bale, A.B., Britt, L.K., Klein, H.H., Siebrits, E., and Dang, X. 2001. Layered Modulus Effects on Fracture Propagation, Proppant Placement, and Fracture Modeling. Paper SPE 71654 presented a the SPE Annual Technical Conference and Exhibition. New Orleans, Louisiana, 30 September – 3 October.

Teufel, L.W. 1979. An Experimental Study of Hydraulic Fracture Propagation in Layered Rock. Phd Dissertation. Texas A&M University. College Station, Texas.

Teufel, L.W. and Clark, J.A. 1984. Hydraulic Fracture Propagation in Layered Rock: Experimental Studies of Fracture Containment. SPEJ 24:19-32.

Van Eekelen, H.A. 1982. Hydraulic Fracture Geometry: Fracture Containment in Layered Formations. SPEJ 22: 341-349.

Voegele, M.D., Abou-Sayed, A.S. and Jones, A.H. 1983. Optimization of Stimulation Design through the Use of In-Situ Stress Determination. JPT 35:1071-1081.

Warpinski, N.R., Northrop, D.A., Schmidt, R.A., Vollendorf, W.C., and Finley, S.J. 1981. The Formation Interface Fracturing Experiment: An In Situ Investigation of Hydraulic Fracture Behavior Near a Material Property Interface. Sandia National Laboratories Report SAND81-0938. June 1981.

Warpinski, N.R., Fnley, S.J., Vollendorf, W.C., O’Brien, M., and Eshom, E. 1982. The Interface Test Series: An In Situ Study of Factors Affecting the Containment of Hydraulic Fractures. Sandia National Laboratories Report SAND81-2408. February 1982.

Warpinski, N.R., Schmidt, R.A., and Northrop, D.A. 1982. In Situ Stresses: The Predominant Influence on Hydraulic Fracture Containment. JPT. 34:653 – 664.

Warpinski, N.R. and Teufel, L.W. 1987. Influence of Geologic Discontinuities on Hydraulic Fracture Propagation. JPT 39-1: 209.

Warpinski, N.R., Lorenz, J.C., Branagan, P.T., Myal, F.R., and Gall, B.L. 1993. Examination of a Cored Hydraulic Fracture in a Deep Gas Well. SPE Production & Facilities 8-3:150.

Warpinski, N.R., Branagan, P.T., Peterson, R.E., and Wolhart, S.L. 1998 An Interpretation of M-Site Hydraulic Fracture Diagnostic Results. Paper SPE 39950 presented at the SPE Rocky Mountain Regional/Low Permeability Reservoirs Symposium, Denver Colorado, 5-8 April.

Weijers, L., Wright, C., Mayerhofer, M. and Cipolla, C. 2005. Developing Calibrated Fracture Growth Models for Various Formations and Regions across the United States. Paper SPE 96080 presented at the SPE Annual Technical Conference and Exhibition. Dallas, Texas, 9-12 October.

Wright, C.A., Weijers, L., Davis, E.J. and Mayerhofer, M. 1999. Understanding Hydraulic Fracture Growth: Tricky but Not Hopeless. Paper SPE 56724 presented at the SPE Annual Technical Conference and Exhibition. Houston, Texas, 3-6 October.

Zhang, X., Jeffrey, R.G., and Thiercelin, M. 2007. Effects of Frictional Geological Discontinuities on Hydraulic Fracture Propagation. Paper SPE 106111 presented at the SPE Hydraulic Fracturing Technology Conference. College Station, Texas. 29-31 January.

Fracture Growth in Layered and Discontinuous MediaFracture Growth in Layered and Discontinuous Media . Norm Warpinski Pinnacle – A Halliburton Service . The statements made during

Documents